IANAL and I am sure Matt will answer with Mongoose's POV but IIRC complete rights means just that: All rights gone, forever. This may be limited by some restrictions; I am fairly sure under German (I live in Germany) laws you can't sign away certain rights (such as being credited for your work) and this might be European law, but I can't be sure.
Personally, I would never, ever, consider signing away "complete rights" for anything. I figure if something I create is good enough for someone else to want to publish it, then it's axiomatically too valuable to give away completely. Of course I also don't hope to make a living writing so my viewpoint may be skewed.
Do keep in mind that anything you create based on other people's work - which would presumably cover the entirety of the 3I - may not be entirely your own property to begin with. And also keep in mind that pure game rules are not protected under copyright (in the US as well as the EU, if memory serves me right) anyway.
I do realize you are probably aware of this, but you should definitely ask an attorney if you have legal questions like this. Copyright is a complex matter. Nobody else - Certainly not I and presumably including Matt, no offense - is qualified to really advise you on such matters.
